The use of light emitting diodes (LEDs) for automotive vehicles have become increasingly popular. LEDs are electronic devices that emit light when activated with an appropriate current as a result of electrons recombining with electron holes within the device, releasing energy partially in the form of photons. LEDs offer many advantages over conventional automotive light sources including lower energy consumption, longer lifetime, improved physical robustness, and smaller size.
Circuits incorporating a diode, an inductor and a switch can be arranged to provide a buck circuit, a boost circuit, or a buck-boost circuit. In a buck circuit (also referred to as a step down circuit) the input voltage is greater than the output voltage, and both are greater than zero. In a boost circuit (also referred to as a step up circuit) the output voltage is greater than the input voltage, and both are greater than zero. In a buck-boost circuit (also referred to as a step up and step down circuit) the input and output have opposite polarities and the absolute value of the output voltage can be greater or smaller than the absolute value of the input voltage.
Advanced Steerable Beam (ADB) automotive headlights are able to direct the light beam to a desired direction with a specified intensity. These systems usually incorporate many LEDs with the possibility of Pulse Width Modulation (PWM) dimming for each individual LED. PWM dimming changes the LED light output by varying the duty cycle of a constant current in the string to effectively change the average current in the string. In typical ADB front headlight systems for vehicles there are several LEDs connected in series, referred to herein as a LED string, which is driven by a constant current driver. Each individual LED is dimmed via a dedicated bypass switch using a PWM method. The constant current driver is capable of operating with a wide Direct Current (DC) input voltage range with adaptive selection of the mode. The mode selection criterion keeps the voltage across the current source stage constant to increase its efficiency. This method of operation maximizes the efficiency of the whole system based on the existing conditions, i.e. the number of active LEDs in the string and the input DC voltage. Due to the ability to select the optimum voltage in all conditions, the system can operate efficiently at high frequency (for example 100 kilohertz (kHz)<f<800 kHz) and be compact i.e. a high power density.
Typically, the LEDs are connected in series as a string. In parallel to each individual LED there is a dedicated bypass switch controlled by the LED string controller block (also known as a matrix manager). If the switch is open, the current flows through the LED and it emits light, and when the switch is closed, the voltage across the LED is less than its operational threshold so the LED is turned off. By selecting the relative duration of on and off intervals, the average brightness of each LED can be individually controlled.
A common method of implementing a current source block is to use a step-down topology and use feedback to make the output current constant. In one conventional technique, a current source circuit based on a buck topology includes a controller circuit that monitors the current passing the switch and selects the proper frequency and duty cycle to regulate the LED string current.